Light Emitting Diodes (LEDs) are increasingly being adopted as general illumination lighting sources due to their high energy efficiency and long service life relative to traditional sources of light such as incandescent, fluorescent and halogen. Each generation of LEDs are providing improvements in energy efficiency and cost per lumen, thus allowing for lighting manufacturers to produce LED light fixtures at increasingly competitive prices. One differentiator for LEDs over the traditional sources of light is their ability to be controlled very precisely relative to other lighting technologies, with the ability to switch on and off in microseconds.
The intensity of the LEDs within LED lighting fixtures may be adjusted using Pulse Width Modulation (PWM) (i.e. changing the time that the LEDs are activated) or by modifying the current that flows through the LEDs. Some LED systems interoperate with independent light sensors that are operable to continuously detect the local light level. In some cases, the LED lighting fixture will have a desired light level (possibly full intensity or a particular dimmed level set by a user) and may adjust the intensity of the LEDs until the light level detected by the light sensor reaches this desired level. In this system, daylight that may be radiating through windows and/or light from other light sources can be used to reduce the required intensity of light from the LED lighting fixture (generally called “daylight harvesting”), hence reducing overall energy usage required to achieve the desired light level within a room. One problem with this type of daylight harvesting implementation is that it requires the independent light sensor which may be an expensive added component to the system. In this type of architecture, it would be difficult to implement the light sensor within the lighting fixture since the light from the lighting fixture would dominate the light detected by the light sensor.
In some implementations, such as the system disclosed within U.S. Patent Application No. 2010/0171442 by Draper et al., herein incorporated by reference, the lighting system integrates a light sensor into or proximate to the lighting fixture itself and attempts to directly detect ambient light levels (i.e. light coming from other light sources other than the lighting fixture itself) during times in which the LEDs are turned off or reduced in power. In these implementations, the controller may sense the ambient light during a time period of the LEDs in their duty cycle in which they are off and then can adjust the current to dim the light intensity in response to the sensed ambient light as it compares to target data. U.S. Patent Application No. 2010/0171429 by Garcia et al. discloses a similar LED system in which ambient light levels are detected during periods of time in which LEDs are turned off.
There are considerable problems with these integrated systems that include light sensors within LED light fixtures and adjust the intensity of the LEDs based upon ambient light levels sampled in very short time periods (ex. one millisecond) while the LEDs are turned off in a duty cycle. The reality of most environments is that the sources of ambient light are often not consistent in their level of light output and may be relatively unstable when sensed within such a limited time period. In particular, fluorescent and neon lights produce oscillation lighting which has dynamically changing light level outputs that may or may not be perceptible to the human eye. Further, other LED lighting fixtures in the same environment may operate using PWM signaling and have periods of time within each duty cycle in which the LEDs are activated and other periods of time in which the LEDs are deactivated. Yet further, video monitors and televisions that may operate within the environment of the LED lighting fixture may have significant changes of their light level outputs due to changes in the content being displayed in a particular moment of time. Also, use of an infrared remote in proximity to the light sensors may increase a sensed light level at the light sensors temporary. Even natural sources of light such as lightning can cause significant temporary changes in a sensed light level at the light sensors.
These inconsistent sources of ambient light would lead to very different light levels being sampled with each sample within the systems of Draper and Garcia. For instance, in any particular moment, the light level sensed at the light sensor may be significant due to the LEDs within another local LED lighting fixture being activated, the oscillation of a neon or fluorescent lighting fixture providing a high light level, an infrared remote being used, etc. The next moment, the light level sensed at the light sensor may be relatively low due to the LEDs within the other local LED lighting fixture being deactivated, the oscillation of the neon or fluorescent lighting fixture providing a low light level, the infrared remote not being used anymore, etc. These fluctuations in detected light levels may lead directly to large fluctuations in the intensity of the LED lighting fixture as the controller adjusts the intensity of the light fixture in direct response to each of these detected ambient light levels. These fluctuations can cause significant flicker issues within the LED lighting fixture that would likely be perceivable to the human eye and could disturb the user of the lighting fixture.
Against this background, there is a need for solutions that will mitigate at least one of the above problems, particularly allowing LED light fixtures to adjust intensity levels consistently and smoothly in response to ambient light levels.